One of serious problems concerning vehicles using fossil fuel, such as gasoline or diesel oil, is release of air pollution. In order to solve this problem, technologies to use a secondary battery, which is rechargeable, as a power source for vehicles have been proposed. Electric vehicles (EV), which are powered only by batteries, and hybrid electric vehicles (HEV), which use batteries and existing engines jointly, have been developed, and some of the electric and hybrid electric vehicles are now in commercial use. Nickel metal hydride batteries are mainly used as the secondary battery, which is the power source for the electric vehicles (EV), and the hybrid electric vehicles (HEV). Recently, the use of lithium-ion batteries as the secondary battery has been studied.
The secondary battery must have high output and large capacity in order for the secondary battery to be used as the power source for the electric vehicles (EV), and the hybrid electric vehicles (HEV). To this end, a plurality of small secondary batteries (unit cells) are connected in serial or parallel with each other to form a battery group, and a plurality of battery groups are connected in serial or parallel with each other to form a battery pack.
However, the high-output, large-capacity battery has a problem in that a large amount of heat is generated in the course of charge and discharge. If heat generated from the unit cells in the course of charge and discharge is not efficiently removed, heat is accumulated, and thereby, the unit cells are deteriorated.
When the temperature of the battery pack is excessively lowered, many reaction-retarding elements affect the electrochemical reaction in the unit cells, and therefore, the performance of the battery is remarkably lowered.
Consequently, temperature control for efficient operation of the unit cells is required for the battery pack, which is a high-output, large-capacity battery.
According to a conventional process for controlling the temperature of the battery pack, an efficient range of temperature Tmg of the battery, a maximum acceptable temperature Tmax of the battery, and a minimum acceptable temperature Tmin of the battery are preset. When the temperature of the battery pack is equal to or greater than the maximum acceptable temperature Tmax, a cooling fan is operated, and, when the temperature of the battery pack is equal to or less than the minimum acceptable temperature Tmin, a heater of an air conditioning system is operated, to maintain the temperature of the battery pack at the efficient range of temperature Tmg. In some of the conventional arts, the temperature section between the efficient range of temperature Tmg and the maximum acceptable temperature Tmax and the temperature section between the efficient range of temperature Tmg and the minimum acceptable temperature Tmin are subdivided to control the driving rate of the fan.
However, the conventional process for controlling the temperature of the battery pack has the following problems.
First, only change in temperature of the battery pack is detected to operate the fan while the cooling or heating efficiency of air supplied by the operation of the fan is not considered. As a result, the efficiency of temperature control according to the operation of the fan is remarkably low. For example, when the temperature of air introduced by the operation of the fan is high, the cooling effect is low although the driving rate of the fan is high.
Secondly, the deterioration of battery efficiency due to the temperature difference in the unit cells constituting the battery pack is not controlled. Although one of the principal causes lowering the operation efficiency of the battery system is the great temperature variation between the unit cells, this temperature variation is not considered in the conventional process for controlling the temperature of the battery pack.